1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method that synthesize, for the purpose of preventing illegal forgery and information leakage by the copy of an important document, a copy-forgery-inhibited pattern image on a document to output the document.
2. Description of the Related Art
There may be a case in which a document such as a receipt, security, or certificate is printed so that, in order to prevent the document from being copied easily, a special pattern such as characters or an image is printed on the background. The entirety of this special pattern is generally called as a copy-forgery-inhibited pattern image including a latent image part and a background part. This is a pattern having a mechanism through which an original is prevented from being easily copied. An original having this pattern realizes an effect to psychologically prevent an attempt for forging an original by copying the original.
A copy-forgery-inhibited pattern image in an original consists of a region in which dots remains after the original is copied and a region in which dots disappear after the original is copied. These regions have the same density. These two regions have substantially the same average density. When these regions are seen by the unassisted eye in a macroscopic manner, it cannot be recognized that these regions include characters or an image (e.g., “copy”). However, the respective two regions have different characteristics when being seen by the unassisted eye in a microscopic manner. In the present invention, these hidden characters or image are differentiated from the background by being called as a latent image.
For example, a region in which dots remain after an original is copied (which will be called as a latent image part) is composed of concentrated dots while a region in which dots disappear after an original is copied (which will be called as a background part) is composed of dispersed dots. As a result, two regions that have substantially the same density and that have different characteristics as described above can be prepared.
Concentrated dots and dispersed dots can be generated, from an image processing viewpoint, by a halftone dot processing using halftone dots having different Lines Per Inch (LPI) or a dither processing using a dither matrix having a different characteristic.
In the halftone dot processing, halftone dots having a low LPI may be used in order to obtain an arrangement of concentrated dots and halftone dots having a high LPI may be used in order to obtain an arrangement of dispersed dots.
In the dither processing using a dither matrix, a dot concentrated-type dither matrix may be used in order to obtain an arrangement of concentrated dots and a dot dispersed-type dither matrix may be used to obtain an arrangement of dispersed dots.
Thus, when the halftone dot processing is used to generate a copy-forgery-inhibited pattern image, a latent image part is appropriately formed by the halftone dot processing having a low LPI while a background part is appropriately formed by the halftone dot processing having a high LPI. When the dither processing is used to generate a copy-forgery-inhibited pattern image, a latent image part is appropriately formed by the dither processing using the dot concentrated-type dither matrix while a background part is appropriately formed by the dither processing using the dot dispersed-type dither matrix.
Generally, a copying machine has limitation in the image reproduction capability that depends on an input resolution for reading the minute dots of a to-be-copied item and an output resolution for reproducing minute dots. Thus, when a to-be-copied item includes isolated and minute dots that exceed the limit of the image reproduction capability of the copying machine, the minute dots cannot be perfectly reproduced in a copy, thus failing to reproducing a part of the isolated and minute dots.
Due to this reason, when a copy-forgery-inhibited pattern image includes a background part that is provided to exceed the reproduction limit of a copying machine, large dots (concentrated dots) in the copy-forgery-inhibited pattern image can be reproduced. However, small dots (dispersed dots) cannot be reproduced, thus causing a hidden image (latent image) to appear. Although the dispersed dots in the printed copy are not entirely disappear, the dispersed dots have a density that is clearly different from that of the concentrated dots. In this manner, the hidden image (latent image) can be visually recognized by the human eye.
With regard to the copy-forgery-inhibited pattern image, a technique called as “camouflage” is also well-known that makes it difficult to determine a hidden character or image (latent image).
The camouflage technique provides or adds, in or to the entire copy-forgery-inhibited pattern image including a latent image part and a background part, another pattern section that has a different density from those of the latent image part and the background part and that has a more minute pattern than that of the latent image part. When this camouflage pattern image is seen from a macroscopic viewpoint, the comparison between the camouflage section and the latent image part or the background part is stronger than the comparison between the latent image part and the background part. This provides an effect in which the camouflage pattern image is conspicuous to further obscure the outline of the latent image or the latent image.
When a copy-forgery-inhibited patern image including the camouflage pattern image is compared to a copy-forgery-inhibited pattern image having no camouflage pattern image, the copy-forgery-inhibited pattern image including the camouflage pattern image provides an decoration effect to a printed material.
In order to allow the latent image after a copy operation to be easily determined, dots in the camouflage pattern image after a copy operation are preferably disappear as many as possible. In the case of the simplest implementation, the camouflage can be realized by not printing dots in a part corresponding to a camouflage pattern image part. When a part surrounding the no-dot-print part is composed of a latent image part and a background part, this part is of course printed with different dots corresponding to the respective parts.
As described above, the copy-forgery-inhibited pattern image has been briefly described.
Conventional copy prevention has been performed by using copy prevention papers sold by printing paper manufacturers. The copy prevention papers have been manufactured by previously printing, on an exclusive paper, a copy-forgery-inhibited pattern image including characters or an image (latent image (e.g., “copy”). These copy prevention papers have been purchased by public offices and companies in order to print a document for which the originality is desired to be guaranteed on a copy prevention paper so that the printed material of this document can be prevented from being copied.
However, the conventional copy prevention paper is prepared by the printing paper manufacturer by preprinting a copy-forgery-inhibited pattern image on an exclusive paper. This has caused a disadvantage such as the cost for the exclusive paper and the cost for preparing preprint papers in an amount larger than needed.
However, a new technique has been recently realized by which an copy-forgery-inhibited pattern image is prepared by software to output, by a laser printer, such a document that includes the copy-forgery-inhibited pattern image in the background (this technique will be called herein as an on-demand copy-forgery-inhibited pattern image output method by a printer). Such an apparatus is disclosed, for example, in Japanese Patent Application Laid-open No. 2001-197297.
The on-demand copy-forgery-inhibited pattern image output method by a printer can print a plain paper so as to include a copy-forgery-inhibited pattern image in the background. Thus, this method can print the copy-forgery-inhibited pattern image in the background when required and in a required amount. This eliminates the conventional need to prepare more-than-required copy prevention papers. Therefore, the on-demand copy-forgery-inhibited pattern image output method by a printer can significantly reduce the cost for papers when compared with the conventional copy prevention method using the copy prevention papers.
Furthermore, the conventional copy prevention papers could be available only as the ready-made ones including hidden characters or image (latent image) and the custom-made ones including hidden characters or image (latent image) prepared for a specific user.
When an original including a copy-forgery-inhibited pattern image is copied, a pattern appears in the copied paper, thereby providing a clear differentiation between the original and the copied matter. At the same time, only the copied matter including the pattern also can tell that the copied matter is a copy. Thus, the copied matter can be prevented from being accepted as the original. At the same time, the copy-forgery-inhibited patern image also provides an effect for psychologically preventing the copied matter from being used as the original. However, an copy-forgery-inhibited pattern image is generated as a dot pattern that is synthesized over the entirety of an image of the contents. This has caused a disadvantage that it is impossible to secure a region for representing information for the outputted matter (e.g., information for determining information for an output device or a person performed the output) or digital information (e.g., information for an electronic signature).
Although the above-described information can be added in the outputted matter by partially synthesizing a two-dimensional bar code or the like into the contents image, this requires the copy-forgery-inhibited pattern image to be controlled in order to prevent the copy-forgery-inhibited pattern image from being superposed with the two-dimensional bar code. Thus, a processing for generating the copy-forgery-inhibited pattern image and a synthesizing processing are very complicated. Furthermore, when a printed matter partially includes a region having no copy-forgery-inhibited pattern image, the appearance is deteriorated.